An important problem in modern society is to make the maximum use of all available materials, including the reuse of scrap materials back into useful products. One component of the modern emphasis on recycling materials is the use of scrap materials in the production of particle board.
The customary method for the manufacture of particle board is to use wood in the form of sawdust, wood particles and fibers which have been defibrated by a mechanical or chemical defibrating process. These materials are typically mixed with a form of adhesive or binder. There are three general catagories of binders commonly in use. Formaldehyde-based binders are one category of binder commonly used, although this material is under increasing scrutiny due to the toxicity of the underlying formaldehyde compounds. A second customary form of binder material in common use are polyurethane-type binders, including under this general term specific compounds such as polymeric diphenylmethane diisocyanate and related compounds generally referred to as "MDI". The third general category of binder involves phenolic-based binders. The phenolic-based binders tend to be more costly, but offer quality advantages in particle boards of higher value.
The customary process of manufacturing particle board involves the blending of the binder with the wood or wood-related particles into a substantially dry blend in which typically about 6% by weight would be binder. This blend is deposited onto a moving mat or belt (hereinafter "belt" for simplicity) either by an "air laying process" or laid onto the belt following wetting; both processes well known in the field. The material following deposition is typically held in place on the belt by means of a vacuum suction applied from below the moving belt, which is typically made to be porous for precisely this purpose. Typically, to manufacture particle boards of three-fourths inch, a deposition on the belt of up to 15 inches may be required. The final particle board is manufactured by pressing the material deposited on the belt into the final particle board with the required density. Commonly, the material will be pressed in a two-step process. A prepressing applied to the material on the mat may be used to reduce the size (that is, thickness) of the material prior to pressing to final density. Pressing to final density then follows, often accompanied by simultaneous heating of the material. Both continuous pressing as well as pressing in a batch process are used in the industry. The density of the final particle board produced by this process is predetermined when material is deposited on the moving belt by adjusting the amount of material (per square foot) deposited on the moving belt prior to pressing.
An alternative process for the manufacture of particle board is to use an extrusion process. In the manufacture of extruded particle board, the starting material is likewise wood, wood-related or other lignocellulosic products blended together with a binder or other substances which impart desirable properties to the final product. For extruded particle board, however, it is not necessary to include in the blend of starting materials a refined wood or lignocellulosic fiber. Instead, it is common in the production of extruded particle board to include plastic (typically scrap plastic) in the blend of starting material. The amount of plastic may vary somewhat from process to process, but typically an equal 50%-50% blend of plastic and wood products (by weight) are included in the starting material for extrusion. The inclusion of plastic imparts a viscosity to the starting blend facilitating extrusion through a die. This process is most useful for the production of long lengths of material with typical cross-sectional dimensions much less than their typical length dimensions. Such material may include floor boards, planking, moldings, window and door frames, etc. Essentially any plastic material which may itself be formed by an extrusion process can be employed along with wood in the manufacture of extruded particle board.
The production of extruded particle board typically does not involve the addition of binders to the starting blend (the plastic acting as the binding agent in this sense). However, "coupling agents" for binding the wooden material to the plastic are required to be included in the blend for extrusion. Coupling agents are essentially materials for the binding the wood and wood fiber to the plastic as the typical wood tends not to bind directly to the plastic without the assistance of special materials for facilitating the bonding. A common coupling agent is commercially available from E. I. du Pont under the tradename "Epolene".
This customary approach to production of particle board by means of extrusion described above successfully gives a material with a good surface appearance. However, the product produced thereby suffers from an important drawback in that the physical properties more closely resemble those of a filled plastic than those of particle board. Due to the relatively large proportion of plastic contained in the material (typically approximately 50%), the properties of the final material are rather like those of a plastic filled with an additive rather than those of a particle board. Thus, it is perhaps misleading to refer to such products as "extruded particle board." Rather a more correctly descriptive name would be "extruded plastic with wood filler".
Thus, the common goal in the production of extruded particle board is to reduce the amount of plastic which is included in the particle board as much as possible. The use of less plastic filler along with an increased proportion of wood material tends to produce a more economical product with more desirable physical properties, more nearly resembling those of particle board. The problem heretofore has been in reducing the mount of plastic material included in the blend while maintaining the extrudability of the material. A reduction of the amount of plastic tends to reduce the extrudability of the product, especially in the surface finish. It has proven to be a serious challenge to reduce the proportion of plastic materials used in the blend while still extruding a product with a good surface finish. The typical problem has been that the extrusion with less plastic results in a rough surface textures in the product.
The present invention demonstrates a blend of wood product and plastic such that the resulting product is capable of extrusion with good surface finish. As described below, the present invention demonstrates an extruded particle board having much less plastic material than in previous products (typically down to 20% plastic by weight), along with appropriate binders and coupling agents, while maintaining extrudability and good surface finish.